Push-pull cable assemblies are well known in the prior art. They are conventionally used to control the operation of valves and other mechanisms from a conveniently located position remotely located from the actual mechanism being operated. For example, when designing a pressurized potable water supply for installation in large commercial passenger aircraft, it is desirable to keep the total length of the individual fluid conduits as short as possible in order to minimize weight. Furthermore, the shorter the fluid conduit, the less energy is required to pump the fluid through the system. Additionally, the probability of leaks occurring somewhere in the system will be diminished to the extent that the system can be reduced in size and capacity. As a result of such considerations, the several valves which are commonly required in such systems to seal off the portions of the system from one another or from the external environment are preferably located in the immediate vicinity of the fluid reservoirs and other plumbing subsystems with which they are functionally associated, rather than having their location determined solely by ease of access during normal operation and maintenance. Thus, it is desirable that it be possible to operate a control valve from a remote location by means of a simple, convenient and reliable mechanism for transferring motive forces. For such purposes, push-pull cable assemblies are commercially available having an operating handle at one end and an actuating rod at the other end connected by a flexible cable contained within a closely fitting sheath. Such as assembly can transmit both tensile and compressive forces from one end to the other. Because such a cable is designed to transmit compressive as well as tensile forces, it does not require the use of any return spring that would increase the force required to pull the cable to its retracted position and which would probably require the use of a brake or detent to prevent the cable from inadvertently returning to its unretracted position, thereby further increasing the operating force required. Since many valves and other mechanisms employ a conical or spherical member that must be rotated through an angle (typically 90.degree.) from one position (e.g., "off") to another position (e.g., "on"), prior art push-pull cable assemblies have been provided with a swivel fitting at the actuating end which permits a swiveling movement of the actuating rod at the end of the cable so that the cable's outer sheath and its associated mountings are not subjected to bending moments as a lever arm operatively connected to the valve's rotating member moves in an arc about the valve's rotational axis and so that the end of the cable will always be able to exert its pushing and pulling motive forces in a straight line and thus will not become bent or kinked.
Typically, a push-pull cable such as described above will be so oriented with respect to the lever arm on the valve such that the "normal" (i.e., when the aircraft is airborne) position of the operating handle on the other end of the cable will be fully pushed-in (or "retracted") and its "alternative" (i.e., its condition during a refilling or draining operation) position will be with the handle pulled-out (or "extended"). Accordingly, to prevent the push-pull cable from being accidentally extended from its normal position to its alternative position, a keeper assembly may be operatively associated with the control handle for maintaining the handle in its retracted position, thereby insuring that it will not be moved to a partially extended position by gravity, vibration or other environmental forces.
With respect to such prior art push-pull cables, it will be appreciated that a certain amount of backlash is associated with their operation, since the fit between the outer surface of the flexible cable and the inner surface of the sheath must be sufficiently loose to prevent any binding even if the sheath is bent and accordingly, when the cable is pushed in, the inner walls of the sheath will tend to force the inner cable into a somewhat spiraled or undulating condition having an effective length shorter then the effective length of the pulled-out cable by an amount known as "backlash". Furthermore, as a result of normal use, the diametrical clearance between the cable and sheath will gradually increase, with the result that a worn push-pull cable will have even more backlash than a new push-pull cable.
When such a push-pull cable is being utilized to control a valve or other similar mechanism provided with a mechanical stop to define the valve's normal position, it will be appreciated that the effective length of the pushed-in cable must be precisely adjusted. When the cable's operating handle is fully retracted and held in place by the keeper assembly, the cable's actuating rod should press against the valve's lever arm so as to hold positively the valve against its stop, while taking up all the backlash between the cable and its sheath; however, there must not be any excessive forces that could distort the valve or its lever or which could subject the cable assembly and its mountings to excessive stresses. The known prior art cable assemblies accordingly provided a backlash adjustment (in the form of an adjustable threaded rod and locknut) which determines the effective overall length of the cable from its handle up to the point at which the cable pushes against the valve's lever arm.
It will be appreciated that such a prior art cable assembly was difficult to adjust since the backlash in the cable had to be determined by trial and error techniques, with one mechanic operating the handle and a second mechanic trying different positions of the threaded rod and locknut. If the adjustment were made too long, the various components would be overstressed and the weakest member would frequently be broken (for instance, the telescoping actuating rod at the end of the cable assembly). If the adjustment were made too short, the valve would be free to move away from its normal position, causing the aircraft's water supply to function erratically.
Accordingly, it is a primary objective of the present invention to provide an improved push-pull cable that may be readily adjusted to hold a valve or other similar mechanism in a predetermined normal position against a fixed stop when an operating handle at the other end of the cable is held in its normal pushed-in position by a keeper assembly.
It is a related objective to provide an improved push-pull cable that automatically compensates for changes in the cable's effective length caused by backlash, wear, etc.
It is yet another related objective to provide an improved push-pull cable having a spring operatively connecting the cable and the mechanism being operated to provide automatic compensation for backlash in the cable without requiring excessive operational forces or stresses.
It is an overall objective of the present invention to provide an improved push-pull cable assembly that will positively hold the controlled mechanism in a normal position, that will automatically take up for wear and backlash in the cable, that will prevent accidental overloading of the mechanism, that will remain in adjustment for an extended period of time, and that will be simple and can be adjusted accurately when such adjustment is required.